This is a port of https://github.com/pulkomandy/gembascan for Wemos D1 mini and arduino boards. Instructions are derived from that repository.
It allows to use the GBS8200 RGB to VGA converter in simple line doubling mode, avoiding all the artifacts and problems caused by resampling and 50>60Hz conversion.
The output is progressive scan VGA at 50Hz and 31.5KHz and it works with CPC RGBS signal.
It has been tested with GBS8200 V4.0 (2014.05.15) and V5.0 (2015.01.15)
First of all, you need to disable the built-in controller of the GBS8200. This is done by closing the P8 jumper.
Then, you need to conenct the microcontroller SDA/SCL lines on the P5 connector. Note that you may need to solder pins to this connector as it is usually not populated.
The wiring is as follows:
Wemos > GBS8200
- D1 > P5.SCL
- GND > P5.GND
- D2 > P5.SDA
You can connect P9.+ to microcontroller 5V ONLY if you are powering GBS with 5V. In other case just connect the microcontroller independently.
Do NOT connect 3.3V > P5.VCC
Arduino > GBS8200
- SCL > P5.SCL
- GND > P5.GND
- SDA > P5.SDA
You will need to install Arduino IDE and the repository for ESP8266. Then, select your board (Wemos D1 Mini or whatever)
For a detailed guide you can check https://randomnerdtutorials.com/how-to-install-esp8266-board-arduino-ide/
Once installed, just compile and flash it.
First make sure P8 jumper is connected.
Then connect the video inputs and outputs. For input I use P11 RGBS and GND from the CPC. Plug the power supply to the GBS8200. Nothing happens at this point, this is expected. Connect power to the microcontroller board. It automatically sets the chip up and starts running.
In case of problems with the image you can press reset button on the microcontroller and it will send commands again.
I have tested it on:
- Amstrad CPC 6128
and others reported that it also works on
- Amstrad 6128Plus
- Amstrad CPC
- Thomson TO8
The tolerance on off video timings is currently quite bad (far from what a standard TV would accept). This creates problems with some Amstrad games using hardware scrolling tricks (seen on Relentless for example), and also with ZX spectrum ULA-128 timings. Ways to improve this are being investigated.
Code for this implementation by issalig
Code is based on Launchpad code by PulkoMandy/SHR. https://github.com/pulkomandy/gembascan
Original idea and experiments with Raspberry Pi by dooklink (https://github.com/dooklink/gbs-control)
Digispark Pro C code and register value list converter tool by mybook4 (http://shmups.system11.org/viewtopic.php?f=6&t=52172)
Thanks to mike/zeroteam, the secrets of ZX spectrum video timings have been revealed:
- ULA 48k: 312 lines, 64us/line, 19968us/frame, 50.080128Hz (Spectrum 48K)
- ULA 1 : 314 lines, 64us/line, 20096us/frame, 49.761144Hz (Didaktik)
- ULA-128: 311 lines, 64.281485us/line, 199991.5us/frame, 50.021153Hz (128k/+2/+3)
The 48k ULA is exactly identical to Amstrad CPC/Thomson timings and should not be a problem. However, for some reason the timings for the other variants are a bit off, and this seems to confuse the GBS converter (but also a lot of other devices). Symptom: the first frame (boot screen) will be displayed fine, but then the screen never gets updated.
A set of datasheets was assembled by dooklink. It contains details on most of the chips used on the GBS8200. http://shmups.system11.org/viewtopic.php?f=6&t=52172